Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, PDAs, wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices, or devices operable in an Internet-of-Things (IoT) environment. Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
Micro-electromechanical-system (MEMS) transducers, such as MEMS microphones are finding application in many of these devices. There is therefore also a continual drive to reduce the size and cost of MEMS devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes. In the case of output transducers, the membrane is moved by electrostatic forces generated by varying a potential difference applied across the electrodes.
To provide protection the MEMS transducer will typically be contained within, or may itself form a package. The package effectively encloses the MEMS transducer and can provide environmental protection and may also provide shielding for electromagnetic interference (EMI) or the like.
FIG. 1 illustrates an example of a MEMS microphone package 100, and in particular a chip scale (CS) MEMS transducer package, also known as a wafer level package (WLP). A MEMS transducer 103, for example a flexible membrane, is attached to a first surface of a package substrate 101. The MEMS transducer 103 may typically be formed on a semiconductor die by known MEMS fabrication techniques. The package substrate 101 may be silicon or PCB or any other suitable material. A cover 105 is mechanically attached to a second surface of the package substrate 101 (and possibly electrically connected).
The type of packaging arrangement shown in FIG. 1 may be referred to as a “bottom port” configuration, wherein the MEMS transducer package 100 is “flip-chip” bonded to the next level of interconnect, for example to a host substrate 111 (e.g. PCB) within a product device. In such a mounting arrangement the host substrate 111 may comprise a sound port 113 to allow passage of acoustic signals to/from the MEMS transducer 103. The sound port 113 may be arranged to substantially line up with the MEMS transducer 103. The sound port 113 acoustically couples to a first volume on one side of the MEMS transducer 103, which may sometimes be referred to as a front volume. It is noted that other bottom-port arrangements may comprise alternative paths for channeling acoustic signals to/from the MEMS transducer 103, in place of the sound port 113.
The package substrate 101 comprises a first cavity 115, with the cover or cap 105 of this example also comprising a second cavity 117. The first and second cavities 115/117 form what is referred to as a back volume. The back volume may be filled with air (or some other fluid or gas), and is sized sufficiently so that the MEMS transducer 103, e.g. flexible membrane, provides the desired acoustic response.
The back volume 115/117 is generally required to allow the membrane to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed (although it will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths, such as small holes in the membrane, that are configured so as present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials, for example due to temperature changes. It is noted that in some applications, such as bidirectional microphones, a sound port 104 may also be acoustically coupled to the back volume 115/117, such that the MEMS transducer 103 receives acoustic signals via both the front volume and the back volume.
Although not shown, the package substrate 101 may comprise integrated electronic circuitry, for example integrated electronic circuitry provided for operation of the transducer, which may for example be a low-noise amplifier for amplifying the signal from a MEMS microphone. Such integrated electronic circuitry is electrically connected to electrodes of the transducer 103 and is also attached to the first surface of the package substrate 101, for example to one or more bonding structures 107, which are provided for mechanical and/or electrical connection to another device, e.g. an associated host substrate 111 of a consumer product in which the MEMS package is being used.
The MEMS package 100 may further comprise a sealing element 109 coupled to the first surface of the package substrate 101, the sealing element 109 surrounding the MEMS transducer 103. In one example the sealing element 109 comprises an acoustic sealing element, such as an acoustic sealing ring. Other shaped sealing elements may also be used. The sealing element 109 is provided for acoustically sealing the MEMS transducer 103, for example such that the MEMS transducer 103 only receives acoustic signals being channeled to/from the MEMS transducer 103 in a bottom-port configuration via the sound port 113.
Thus, the sealing element 109, which may be a metalized ring, i.e. a metalized annular bonding structure, is provided to aid in forming an acoustic channel in an assembled host device.
The package substrate 101 and associated substrate 111 to which the MEMS transducer package is affixed in an assembled host device may have different thermal expansion characteristics. As a consequence, changes in temperature can lead to the package substrate 101 expanding at a different rate to the associated substrate 111, and since the package substrate 101 and associated substrate 111 are mechanically fixed by one or more bonding structures 107, this can result in an acoustic seal formed by the sealing element 109 being broken.